This invention relates to a method for the production of a cylinder block for an internal combustion engine for motor vehicle. More precisely, this invention relates to a method in which at least one cylinder liner is arranged in a mould and aluminium-based material is cast into the mould and cooled so that the cylinder liner is incorporated in the cylinder block.
In a known solution, the cylinder liners are made of cast iron, or other material, which melting point is considerably higher than that of the aluminium-based material forming the cylinder block. In this case, there is no risk of damaging the cylinder liners while casting the material destined, once solidified, to form the cylinder block.
Technically more advanced solutions envisage the use of cylinder liners made of aluminium-based material, such as, for example, hypereutectic aluminium alloys or composite aluminium alloys reinforced with ceramic particles (silicon carbide, alumina, ceramic fibres, etc.). With reference to cast iron cylinder liners, aluminium liners present the advantage of being lighter in weight and permitting the construction of an engine which consumes less oil, because the aluminium liners have an expansion coefficient which is much closer to that of the aluminium pistons. For this reason, during operation of an internal combustion engine fitting aluminium liners, the variation in play between the piston and the internal surface of the respective cylinder liner is lower.
The production of cylinder blocks with cylinder liners incorporated by casting poses various problems. The cylinder liners usually produced by extrusion normally present an external layer of alumina due to the oxidation of the aluminium-based material in the presence of oxygen. A metallurgic bond between the cylinder liners and the cylinder block cannot be obtained if cylinder liners coated with a layer of alumina are incorporated in the molten material which is destined to form the cylinder block, because the alumina layer(which fusion temperature is considerably higher than that of the aluminium alloys) forms a barrier between the cylinder liners and the material forming the cylinder block. With the alumina barrier, the bond between the cylinder liner and the cylinder block is only mechanical, while a metallurgical bond concerning at least of a part of the contact surface between cylinder liner and cylinder block would be desirable.
The applicant experimented a method consisting in removing the alumina layer by turning immediately before arranging the cylinder liners in the mould. However, this method introduces an additional cost related to the turning process. Furthermore, experimental experience demonstrates that without the alumina layer the cylinder liners can be damaged when the molten material destined to form the cylinder block is cast. Specifically, the formation of localised fusion of the cylinder liners was found near the mould casting channels.
The purpose of this invention is to describe a method for the production of a cylinder block with aluminium liners which are not effected by said problems and which permits the creation of a metallurgic bond along at least a part of the contact surface between liner and cylinder block.
According to this invention, this purpose is reached by means of a method which characteristics are described in the main claim.